In recent years, increasing focus has been given to wireless communication between different communication and computational devices using Wireless Local Area Networks (WLANs). One such WLAN technology is High Performance Radio Local Area Network Type 2 (HiperLAN2), which has been standardised by the European Telecommunications Standards Institute (ETSI). HiperLAN2 enables wireless, communication of very high data rates (up to 54 Mbps) over short ranges. HiperLAN operates in the 5 GHz frequency range, uses Orthogonal Frequency Division Multiplex (OFDM) transmission techniques and supports different Quality of Service (QoS) for different connections. HiperLAN2 may thus carry different services including various data services, voice or video services.
In HiperLAN2, mobile terminals communicate with an access point which is typically connected to a fixed network and/or other access points. The specification for the physical layer of HiperLAN2 is specified in ETSI Technical Specification TS 101 475, which specify that a precise receive power measurement and precise transmit power control must be implemented. Specifically, TS 101 475 specifies that the mobile terminal must control the transmit power such that the transmitted signal is received at a given level at the access point independent of the distance between the access point and the mobile terminal. To enable this, the access point broadcasts information about the transmit power level it uses, as well as the given receive level it expects. This information is used by the mobile terminal to calculate the transmit power by assuming that the path loss from the access point to the mobile terminal (downlink) is approximately equal to the path loss from the mobile terminal to the access point (uplink). Hence, the transmit power of the mobile terminal can be determined asPTransmit,MT=PReceive,AP+PTransmit,AP−PReceive,MTwherein PTransmit,MT is the required transmit power of the mobile terminal to meet the given receive level at the access point, PReceive,AP is the given receive level at the access point, PTransmit,AP is the transmit power level of the access point, PReceive,MT is the received level at the mobile terminal of the transmission from the access point and all values are measured in dBm. Hence, in order to meet the specification, the mobile terminal must be able to accurately measure PReceive,MT and control PTransmit,MT.
With usual semiconductor technology, the transceivers cannot be fabricated such that the transmitted power levels have sufficient precision. Specifically, for HiperLAN the dynamic range of the RF input signal is from −85 dBm to −20 dBm. Thus considerable gain and gain variation are required through the receiver chain. The circuit must measure the received power in 1 dB steps with ±5 dB precision (±8 dB at the ends of the range), which is exceedingly difficult to achieve without calibration. Likewise, the transmitter has to cover a power range from −15 to +23 dBm for the lower Hiperlan2 band, and −15 to +30 dBm for the higher Hiperlan2 band, with power steps of 3 dB and a precision that ranges from ±4 dB (access point at high power) to ±10 dB (mobile terminal at low power). Further due to the high peak to average value, OFDM requires high linearity over a wide dynamic range and meeting the requirements for the transmitter and especially the power amplifier cannot easily be achieved by current technology.
Thus high accuracy is required both of the receiver and the transmitter over a wide dynamic range, and to achieve this it is necessary to calibrate both the receiver and transmitter. Calibration is commonly performed at manufacturing of electronic equipment. However, this has a number of disadvantages including                It does not take into account variations occurring after manufacturing. These variations can be very significant as a consequence of component drift, ageing and temperature variations.        Manual calibration can be very time consuming and thus costly requiring special test setup and measurement circuitry.        The calibration must be maintained throughout the lifetime of the product either by storing calibration values in non-volatile memory or by setting of adjustable components (such as a variable resistor or potentiometer), thus requiring additional components.        
One system of calibration is described in U.S. Pat. No. 6,272,322. In this system a pair of receivers perform a loop back test to determine a relationship between the transmit and receive gain for each transceiver. A path loss between the first transceiver and the second transceiver is computed by transmitting a pair of signals in opposite directions to determine the relationship between the transmit gain of one receiver and the receive gain of the second receiver. The individual transmit gain and receive gain is calculated from this relationship. The system described is relatively complex and specifically requires two transceivers operable to communicate with each other. U.S. Pat. No. 6,118,811 discloses a transceiver that can insert calibration signals of known level and frequency into transmitters for calibration and correction of transmitter parameters. An output of the calibrated and corrected transmitter can then be subsequently coupled into a mixer together with a receiver local oscillator signal and input to a receiver for calibrating and correcting receiver parameters based on the calibrated and corrected transmitter output. Thus an improved system of calibration would be advantageous.